1. Field of the Invention
The present invention relates to a porous hollow-fiber having a multiplicity of fine pores in peripheral walls thereof and having selective separation capabilities. More particularly, the present invention relates to a porous hollow-fiber which is suitable for use in artificial lungs, plasma separation, water purification, and the like.
2. Description of the Related Art
A method of producing a porous film by stretching (drawing) a film of crystalline polyolefin such as polypropylene and forming pores in the film is disclosed in U.S. Pat. No. 3,558,764. In this method, an unstretched film of polypropylene is melted and extruded at a temperature in the range of 10.degree.-40.degree. C. higher than its melting point, and this film is taken up within a take-up ratio (i.e., draft) of 20-180. A film having pores connected to each other therein and having an average pore size of 1,000-2,000 .ANG. is thereby obtained.
Furthermore, a porous polypropylene hollow-fiber and a method of producing the same are proposed in Japanese Patent Publication No. 52123/1981. This publication discloses a porous polypropylene hollow-fiber formed by effecting extrusion at a spinning temperature of 210.degree. C. and taking up the fiber at a draft of 180-600. In the resultant fiber the thickness of a peripheral wall is less than 40 .mu.m, a multiplicity of fine pores respectively connected to the peripheral wall are present, and a distribution curve of the fine pores has at least one maximum point in a range of 200-1,200 .ANG., thus, it is possible to obtain a hollow-fiber which is characterized by its gas permeability.
However, in a conventionally known hollow-fiber, the pores in the peripheral wall are extremely nonuniform, and the distribution of the pore size is large, so that the fiber has a large drawback in terms of the selective separation capabilities. In other words, to obtain a porous hollow-fiber having an excellent capability to effect selective separation, the uniformity of the size of the pores is important.
If a polyolefin such as polypropylene is made porous in a conventional method by being fibrillated by stretching, pores are formed by portions whose configuration and size are nonuniform and which form relatively large and meandering meshes and by relatively slender fibrils (hereafter referred to as fine fibrils) which run in these portions substantially parallel to the longitudinal direction of the hollow-fiber.
Thus, the configuration and size of the pores obtained by the known methods are not uniform, and the aforementioned relatively large mesh-like portions form closed circuits. In other words, these portions which form relatively large meshes run in all directions relative to the longitudinal direction of the hollow-fiber, thereby forming continuous, closed circuits, i.e., meshes, which are of various sizes of a dimension substantially equivalent to the length of the aforementioned fine fibrils.
Accordingly, the length of the fine fibrils varies depending on locations, and the distribution of the pore size is therefore wide. Such a porous hollow-fiber having a wide distribution is inferior in terms of the selective separation capabilities.